The present invention relates to a power amplifier unit and particularly but not exclusively to a power amplifier unit for use in a transmitter for a mobile communications system.
The present invention is concerned with power amplifiers, particularly linearised power amplifiers, that is amplifiers which do not generate distortion products and thus produce output signals containing only the spectral components of the input signal. Outside the input signal spectrum, signals should be attenuated as far as possible. Such amplifiers are used particularly in mobile communication networks, where transmission systems operate with in designated frequency bands which are termed herein transmission bands or communication bands. For example, according to one mobile communication standard, transmission channels each having a band width of 5 MHz may lie within a communication band of 2.11 GHz to 2.17 GHz, that is having an overall frequency width of 60 MHz. Thus, the power amplifier which amplifies these transmission channels prior to transmission via an RF antenna is effective if it amplifies signals in this frequency range, but attenuates signals outside that frequency range.
Generally, linearised power amplifiers have gain characteristics which are dependent on input and output power levels. With constant envelope input signals, such as GMSK used in GSM, it is possible to set the power amplifier to an optimum operational point and to leave it at that point. However, for variable envelope input signals such as wide band CDMA (W-CDMA) or multi-carrier GSM, if the power amplifier is set to a fixed operational point, this will be optimum only for a small proportion of the input signal. Thus, for most of the input signal the power amplifier will be operating with a lower efficiency and linearity than it is capable of delivering at its optimum operational point.
It is an aim of the present invention to improve the operational characteristics of a power amplifier unit.
According to one aspect of the invention there is provided a power amplifier unit comprising: power amplifier means having an input arranged to receive a variable envelope input signal and circuitry for setting an operational point of the power amplifier means; means for detecting the instant input level of the input variable envelope signal to generate a power level information signal; and means for using the power level information signal to control the setting circuitry such that the operational point of the power amplifier varies in dependence on the instant power level of the variable envelope input signal to maintain a constant gain of the power amplifier.
The power amplifier means can be constituted by a power amplifier which includes setting circuitry in the form of gating circuitry such as an FET responsive to a bias control voltage. In that case, the power level information signal is used to generate a bias control signal for controlling the gating circuitry of the power amplifier to set its operational point.
The means for using the power level information signal can comprise a look-up table which holds for a plurality of power levels respective digital control settings. A digital to analogue converter can be provided to convert the digital control settings to an analogue bias control signal for controlling the gating circuitry.
Alternatively or additionally, the power amplifier means can include a power amplifier connected to receive a preamplified signal from a preamplifier which is connected itself to receive the variable envelope input signal. The preamplifier can be controlled by a digital control setting directly from the look-up table or by an analogue signal generated by the digital to analogue converter when present. The power level of the output signal from the preamplifier is thus attenuated according to the instant input level of the input variable envelope signal. This allows the power amplifier itself to operate within its correct operational range. Thus, in this sense the preamplifier acts as setting circuitry for setting the operational point of the power amplifier.
According to another aspect of the invention there is provided a transmission system for a mobile communications network comprising: means for generating an information signal representing data to be transmitted via the mobile communications network; a mixer for combining said information signal with a carrier signal to generate a combined signal for transmission purposes, said combined signal having a variable power envelope; a power amplifier for amplifying said combined signal to a power level adequate for transmission via an RF antenna; and means for setting the operational point of the power amplifier responsive to the instant input power level of the variable envelope input signal such as to maintain a constant gain for the power amplifier.